Expert: Paul Soderman - 4/6/2009

Question
QUESTION: Hi Paul, i race hydroplanes and on our boats we have 2 mechanisms that help steer the boat(http://inboardhydroplanes.com/hydroplanes/anatomy.htm), one is the rudder which is controlled by the steering wheel and the other is what we call a skid fin which is located on the left side sponson and is fixed to the boat and not adjustable while in motion, to help turn the boat, always left turns.the skid fin is installed perpendicular to the water, with a slight angle of 10 to 15 degrees (off of 90 degrees)to increase turning or i guess minimize skidding ,see picture. My question is this, would you know at what angle a fin on a boat would act as a hydrofoil and cause lift, which is what i wish to avoid, or in otherwords at what maximum angle could i adjust my skid fin without it causing lift? I know this is a specific question to my application but any help is appreciated.

thanks

Bruce

ANSWER: Bruce
I don't quite understand two important aspects of your problem.  You say the skid fin is installed perpendicular to the water, which I am going to assume means perpendicular to the water surface.  So it sticks down and is angled to give you a side force that resists skidding.  Correct ?  (Doesn't that slow you down ?.... I guess skidding is worse.)

Now for my second confusion.  You want to avoid lift.  But every airfoil or fin set at angle of attack generates lift.  But in your case the lift is parallel to the water surface more or less, so why does that bother you ?

So, without totally understanding your problem, I can say that a fin at any angle will generate lift.  That lift can be predicted fairly easily if we know velocity, angle of attack, water density (I know that) and airfoil shape.

With clarification I may be able to help more.
Paul  

p.s.  Novice question - why skid ?... why not lift the outside pontoon and carve the turn like a skier ?

---------- FOLLOW-UP ----------

QUESTION: You are correct, the fin acts as a pivot point to help turn the boat faster, as yes it does slow us down but if we did not have the turning fin, the rudder alone would not suffice to turn the boat in a small enough radius to follow the course buoys, the boat would take a very long time to turn, we want to turn as fast as possible and avoid as much "skidding as possible".

The lift on the fin will bother me because we want both sides of the boat to ride evenly, the boat as parallel as possible with the water, therefore if the left sponson (the one that has the fin) has more lift, then on the straightaways this will cause the left side to lift more than the right side causing the right sponson to tap the water more and everytime you tap the water you slow down.

Lastly, when we turn , we call it the skid fin because the turn is a controlled skidding motion across the water due to g-forces, a necessary evil of boat racing, on asphalt with cars the grip is better but race cars always want a certain amount of grip on the tires. We do not want to carve the turn like a skier, actually it is quite the opposite, we do not want the outside sponson touching the water at all, friction causes speed loss, in a perfect turn we want the rudder generating the turn and the fin "hooking up" in the turn without the sponsons hitting the water. This is the fastest way around a turn.

Therefore, as i might of thought, any fin put at an angle ,even 1 degree will in theory cause an x amount of lift. it is for me to determine through trial and error by trying different angles, what affects performance or handling of the boat, correct???

the reason i asked my previous question was because i was told by a hydroplane race boat builder not to put the fin at more than 15 degrees, because at that point the fin would start to act as a hydrofoil. i was trying to find out the theory behind his statement, i guess this is the point he believes will cause the sponson on the fin side to rise?

thank you very much for being interested in my questions.

Bruce

Answer
Yes Bruce, this is very interesting and I am starting to understand.  Your photo suggests that in a turn the side force on the boat is carried by the rudder, skid fin, and part of the the left pontoon.  The total side force is equal to the centrifugal force on the boat (mV^2/R, where m =  mass, V = velocity, R = turn radius).  I assume the pontoon side force is small.  And the rudder force is actually toward the outside of the turn.  So the skid fin force is approximately equal to the centrifugal force minus the rudder force.  These are actually vectors whose directionality must be accounted for.  The rudder and fin forces are proportional to angle of attack, surface area, velocity squared, and water density.  I guess what I am getting at is that an engineer could estimate all your forces and give you a starting point for fin optimization.  Without that your trial and error approach is the best bet.

The boat builder may be right that 15 degrees is too high an angle, but only because the fin might stall and lose lift.  Even 10 degrees might be a maximum depending on the fin shape. I don't understand his comment about hydrofoils because they operate parallel to the water surface to give lift upward. In a turn with the boat tilting left side down the fin will tend to pull the pontoon to the left and down. If you drove hard enough it would draw you right under water.

I don't yet understand why the skid fin causes the left pontoon to lift and the right pontoon to tap the water on the straightaway. It seems more likely that the drag and lift on the skid fin would tend to hold the pontoon down.  That would cause the right pontoon to tap.  Or is it because the fin is not really vertical but angled outboard at the tip ?

Wouldn't it be nice if the skid fin could be moved to zero angle of attack on the straightaways and some optimum angle in the turn.
Good luck.
Paul